As is generally known, a fuel cell system frequently comprises a plurality of fuel cells assembled together to form a fuel cell stack which each have an anode, a cathode and a membrane. The anodes of the individual fuel cells are electrically connected together so that one speaks of the anode side of the fuel cell stack. In the same manner the cathodes of the individual fuel cells are electrically connected together and one speaks here of the cathode side of the fuel cell stack. At the anode side the fuel cell stack has an inlet for a fuel and an outlet for a non-consumed fuel and also for exhaust gases which arise at the anode side. At the cathode side the fuel cell stack likewise has an inlet, either for a gaseous oxidizing agent such as air and an outlet for exhaust gases which arise at the cathode side, with a compressor normally being connected upstream of the cathode side inlet of the fuel cell stack.
It is generally known that the membranes of the individual fuel cells must be kept moist in operation in order, on the one hand, to protect them from damage and, on the other hand, to achieve a high degree of efficiency.
In the operation of a PEM fuel cell (PEM=proton exchange membrane) protons which originate from the hydrogen component of the fuel supplied to the anode side migrate through the humidified membranes and react at the cathode side with the oxidizing agent, which is normally supplied in the form of atmospheric oxygen, and thereby form water with the simultaneous generation of electrical power. Thus water always arises at the cathode side. In the operation of the fuel cell stack a part of this water diffuses through the membranes to the anode side of the fuel cell stack so that both sides of the fuel cell stack are humidified. Since the water which is produced is frequently present in excess it is removed from the anode side and the cathode side and then collected and/or drained off.
Despite this water which is produced in operation, the problem nevertheless remains that the gaseous oxidizing agent supplied to the cathode side and also the fuel supplied to the anode side have to be adequately humidified so that in all operating states of the fuel cell system, for example during startup and with dry environmental conditions, i.e. dry air, the membranes are always kept moist and an adequate humidity is present both at the anode side and also at the cathode side.
The active humidification of the gas flows that are supplied is frequently restricted to the cathode side of the fuel cell system, because this is generally sufficient in order to humidify the membranes and, as already stated above, a diffusion through the membranes to the anode side of a fuel cell system in any event takes place. Up to now relatively complicated devices are known for the humidification of the gas flows which are operated with fully demineralized, i.e. deionized water which originates from the operation of the fuel cell system. Since such humidifying apparatuses are not only complicated but also take up a relatively large amount of space another system has been tested at the applicants premises in which two nozzles are supplied with deionized water from a pressure pump and the nozzles spray water into the induced air stream. It has, however, been shown that droplet formation can occur, in particular on switching on the pressure pump, which can lead to problems. When switching on the pressure pump a sudden pressure loading of the nozzles arises which leads to the droplet formation, with the droplets being able to migrate under some circumstances up to and into the fuel cell stack. Such droplets can have a negative effect on the degree of efficiency of a fuel cell system because they for example block fine flow passages in the area of the cathode. Moreover, it can transpire that a water droplet lands on a temperature sensor provided in the fuel cell system or in the fuel cell stack and cools it down to such an extent that the outlet signal of the temperature sensor simulates a temperature reduction of the fuel cell stack and leads, via the control of the fuel cell system, to an unnecessary and undesired switching off of the fuel cell system.